Electromagnetic (EMF) forming uses very high-current pulses in a specially designed electrical coil to generate magnetic fields, which impart opposing magnetic fields in a highly electrically conductive metal workpiece, such as an aluminum alloy or steel. With the coil held in a fixed position, the repulsive magnetic forces act upon the workpiece causing it to deform at very high strain rates. Metals deformed at these very high strain rates can exhibit “hyperplasticity,” a level of plastic ductility well beyond what the material is capable of during conventional forming, e.g. flanging and hemming, operations.
Roller hemming uses a solid wheel, driven and controlled by a robot (or other device) to gradually bend a 90° flange to a closed hem position as it traverses the perimeter of a panel. The roller hem usually requires two to three passes around the panel to completely bend the flange to the closed, flat hem position.
An advantage of the roller hem method compared to conventional hemming is the alternate strain path through which the flange is bent. Conventional hemmers deform the flange through a “plane strain bending” path, which is very severe and can cause cracking failure when hemming aluminum panels, especially panels stamped from AA6111. The roller hem method imparts a component of strain in the direction of the hem line, different from “plane strain bending,” that allows AA6111 to be flat hemmed without cracking.